1. Field of the Invention
The present invention relates to an intaglio printing method and an intaglio printer suitable for forming a wiring pattern and/or bumps such as bump electrodes on a print receiving material on which printing is to be performed, such as a substrate and a semiconductor package, using a paste or a fused metal.
The present invention also relates to a method of forming the wiring pattern and the bumps such as bump electrodes on a printing substrate on which printing is to be performed, such as a substrate and a semiconductor package, using the intaglio printing method, a method for forming a wiring pattern, an apparatus for carrying out the method of forming the wiring pattern, the bump electrode and the wiring pattern.
2. Discussion of Background
As methods of forming a wiring pattern on a substrate, there are conventionally known a method of performing printing using a stamp and an electroconductive paste; a method of masking a substrate with a stencil and spraying an electroconductive material over the masked substrate; a method of depicting a conductive pattern, using a syringe; a method of depositing an electroconductive material on the concave and convex portions of a substrate and polishing the electroconductive deposited convex portions to form a wiring pattern in the concave portions, and using the concave portion as the wiring pattern; a method of forming a wiring pattern by die-casting an electroconductive material onto concave portions of a substrate; a method of printing a wiring pattern on the surface of a substrate, using a catalyst, and causing a metal to separate therefrom; a method of printing a wiring pattern on a substrate by performing vacuum deposition of a metal on the substrate through a stencil; and a method of forming a wiring pattern by bringing a metal foil into pressure contact with a substrate, using a die in the shape of the wiring pattern, which is heated to high temperature.
The element technologies for the above-mentioned conventional methods are composed of a patterning technology and a film formation technology. Currently, however, the patterning is performed by printing or photolithography, and the film formation is performed by laminating metal foils or plating. Representative examples thereof are subtractive method and additive method.
The subtractive method, which is also referred to as etched foil method, is carried out by performing patterning by etching. This method is currently used most. In this method, a copper foil laminated plate is first fabricated by laminating a copper foil having fine projections on a laminating surface thereof on a substrate such as a glass epoxy substrate with the application of high pressure thereto. The copper foil side of the copper foil laminated plate is subjected to photoresist coating. A photo film with a design of a wiring pattern is then superimposed on the photoresist coated side of the plate, exposed to light and then subjected to development, whereby a photoresist masking pattern is formed on the copper foil laminated plate. The copper foil laminated plate with the photoresist masking pattern is then etched, whereby a wiring pattern is formed on the copper foil laminated plate. This method has the shortcomings that the above-mentioned steps including the art work of the photo film are complicated, high density is difficult to attain in comparison with the additive method which will be described in detail later, and the method is more or less lacking in patterning reliability. Furthermore, a photoresist, a resist releasing agent, an etching agent and others are required as auxiliary materials for this method, so that this method has a further shortcoming that the production cost including the cost for treating waste fluids to be discarded is high.
The above-mentioned additive method is also called plating method. In this method, an adhesive agent is applied to the surface of a substrate such as a substrate made of glass epoxy, and then a catalyst is applied thereto to improve the adhesiveness of a plating. A photoresist is coated thereon and a photo film is superimposed on the coated photoresist, and exposure and development are then performed, whereby a masking pattern is formed. Non-electrolytic copper plating is performed in the areas free of the masking, whereby a wiring pattern is formed. This method is superior to the subtractive method for attaining high density and reliability, but has the shortcomings that the process is complicated and the production cost is high.
For the formation of bump electrodes on a semiconductor package, a larger number of methods have been proposed and used in practice in comparison with the wiring methods of printed wiring boards.
For instance, for the production of BGA, a method of placing in a pad solder balls which are equal in size to bump electrodes, and joining the solder balls to the pad of a package by reflow is currently most used. In this method, solder balls are joined to the circuit pads on the side of the package. In this method, on the side of a solder manufacturer, a fused solder is added dropwise to an oil which is heated above the melting point of the solder, whereby fused solder particles, each of which is substantially in the shape of a true sphere, are formed, and the thus formed fused solder particles are cooled, washed and classified in accordance with the particle sizes thereof. The solder balls for the formation of the bump electrode are thus produced. In a process of forming the bump electrode, using the solder balls, the solder balls are moved onto the pad on the side of the package, using absorption nozzles, reflowed and joined.
In this method, however, the solder balls are expensive, the height of bump electrodes varies in accordance with the variation of the diameter of each bump electrode, so that an open portion is formed in a joining portion when the bump electrodes are mounted on a substrate. Furthermore, the bump electrodes may fall off the pad on the package side due to imperfect fusing of the bump electrodes and the pad on the package side. Cracks are formed in a junction portion between the bump electrodes on the package side and the pad on the substrate side, or the bump electrodes are peeled off the pad, due to the stress caused by the difference in the coefficient of thermal expansion between the substrate and the package.
In addition, it is known that a method of forming solder balls by cutting a thread solder or a ribbon solder with a predetermined length and punching the cut solder into a cylindrical solder, and fusing the solder to form solder balls, or a method of using the punched cylindrical solder as it is and moving the solder onto a pad of a package and subjecting solder to reflow. These methods also have the same shortcomings as mentioned above for the same reasons as mentioned above, such as the formation of the open portion in the junction portion, falling off of the bumps, the formation of cracks in the junction portion between the bump on the package side and the pad on the substrate side, or the peeling of the bump on the package side off the pad on the substrate side.
Furthermore, there is known a method of forming bump electrodes on the pad on the package by printing a solder. As the printing masks used in this method, there are a stencil mask and an intaglio printing mask, and as the solders for use in this method, there are a solder paste and a fused solder. With the combination of such printing masks and the solders, various methods have been proposed. In the method of performing printing using a solder paste, it is necessary to perform reflow after the printing.
In these methods, the diameter of the bump changes in accordance with the variation of the filling amount of the solder into the mask which depends upon the degree of scooping the solder by a squeegee, and also in accordance with the variation of the degree of the passing of the solder through the mask, so that these methods have the shortcoming that open portions may be formed in the junction portion. Furthermore, when the fusing of the pad on the package side and the bump electrodes is imperfect, the bump electrodes will fall off the pad on the package side. Furthermore, cracks are formed in a junction portion between the bump electrodes on the package side and the pad on the substrate side, or the bump electrodes are peeled off the pad, due to the stress caused by the difference in the coefficient of thermal expansion between the substrate and the package. In the method of forming the bump electrodes by printing a cream solder, using the stencil mask, the loss of holes in the mask may occur due to the missing of cavity holes in a photosensitive resin formed by photolithography when plating is performed in the course of the production of a metal mask (that is, an additive mask) which is currently mainly used. This hinders the formation of fine bumps.
As methods of forming bump electrodes by plating, the following three methods are mainly known: A method comprising the steps of masking the circumference of a pad with a thin film of a liquid resist and then forming a bump electrode with electrolytic plating. In this method, the plating tends to spread transversely, so that this method is not suitable for forming bump electrodes with multiple pins. In order to improve the above method, there has been proposed a method comprising the steps of forming deep holes in a thick dry film by photolithography and performing electrolytic plating in the deep holes to secure the plating thickness for the bumps. This method has a shortcoming that the production cost is high. Under such circumstances, there is proposed a method of forming bump electrodes with multiple pins, while securing a sufficient height for the bumps by performing non-electrolytic plating, using a thin liquid resist, in an attempt to reduce the production cost. However, the objects of the method has not yet sufficiently been attained.
Furthermore, a method of forming bump electrodes, using a conventional wire bonding technology for internal bonding of semiconductor packages is also proposed. In this method, balls formed by discharging to a metal thin wire at a tip of a capillary are bonded to a pad as they are to form bump electrodes. This method features how to cut unnecessary wire portions. There are several methods of cutting unnecessary wire portions. Any of the cutting methods, however, has the problem that the height of the bumps cannot be made uniform. In order to solve this problem, a method of leveling the height of the bumps by use of a holding jig or by polishing, and a method of making the balls more spherical by being fused under reflow have been proposed. However, these methods have the shortcomings that bumps cannot be formed at a time and the production efficiency thereof is low and it is difficult to prepare bump electrodes with multiple pins.
As other methods of forming bump electrodes and junction methods for semiconductor packages, there have been proposed a method of forming bump electrodes using a solder or an electro-conductive adhesive agent supplied by a dispenser, a method of forming bump electrodes made of an electro-conductive photosensitive resin by photolithography, a junction method for semiconductor packages, using an anisotropic electroconductive resin, and a method of forming bump electrodes, using coagulation force of super solder. These methods, however, still have problems with respect to the production cost and the quality of the junction.
Thus, the conventional methods have not yet sufficiently attained requests such as (a) the formation of bumps with high density, (b) high production reliability free of the formation of open portions in the junction, the falling off of bumps, the formation of cracks in the junction portion between the bumps on the package side and the pad on the substrate side, or the peeling of the bumps on the package side off the pad on the substrate side, (c) the formation of bumps with high productivity, and (d) the reduction of the cost for the formation of the bump electrodes.